National Rhododendron Gardens is a nice garden in Mount Dandenong, just located east 500 meters from the Olinda township. It's famous of colored blooms of rhododendrons, azaleas, camellias, cherries and daffodils. Spring is the best season to visit. There are also some small gardens, like Japanese Garden, every year when Cherry Blossom days, Japanese-themed festival holding in this area.
For more information, link to Parks Victoria website.